The influence of neighbouring grassland plants on each others' endomycorrhizas and root-surface microorganisms

In three glasshouse experiments the grassland plants Anthoxanthum odorattum. Lolium perenne, Plantayo lanceolata and Trifolium repens were grown in pots of grassland soil, in monocultures and various two-species mixtures. Mixed-species and single-species swards were also sown in a garden bed; and in a permanent pasture P. lanceolata individuals whose surrounding vegetation had been removed were compared with control plants. Abundance of root-surface bacteria and fungi, and of internal mycorrhizal infection were measured by direct observation of randomly-selected root segments.

The abundance of all three microbial groups on a given “host” species was often significantly altered by the presence of other plant species. Bacteria usually changed in the same direction as fungi, but proportionately less. Mycorrhizas usually changed less than the other two groups and not always in the same direction. In the pot experiments, when growth in a mixture resulted in a plant being larger and having higher nutrient concentration than in monoculture, the abundance of fungi on its roots also increased; bacteria and mycorrhizas showed this correlation less clearly. Superimposed on this relationship was a general tendency for fungi, and to a lesser extent bacteria, to be more abundant in mixture than monoculture even when the plants did not change in size. Possible mechanisms involved in these responses are discussed.